Our Galaxy, Host of the Interstellar Medium -- Radiations and Magnetic Fields -- Radiative Transfer and Excitation -- The Neutral Interstellar Gas -- The Ionized Interstellar Gas -- The Interstellar Medium at High Energies -- Interstellar Dust -- Heating and Cooling of the Interstellar Gas -- Interstellar Chemistry -- Photodissociation Regions -- Shocks -- Shock Applications -- Interstellar Turbulence -- Equilibrium, Collapse and Star Formation -- Changes of State and Transformations.Describing interstellar matter in our galaxy in all of its various forms, this book also considers the physical and chemical processes that are occurring within this matter. The first seven chapters present the various components making up the interstellar matter and detail the ways that we are able to study them. The following seven chapters are devoted to the physical, chemical and dynamical processes that control the behaviour of interstellar matter. These include the instabilities and cloud collapse processes that lead to the formation of stars. The last chapter summarizes the transformations that can occur between the different phases of the interstellar medium. Emphasizing methods over results, "The Interstellar Medium" is written for graduate students, for young astronomers, and also for any researchers who have developed an interest in the interstellar medium.